DESCRIPTION: (Applicant's Abstract) This application is a request for continued funding of an Independent Scientist Award (ISA; K02) to support Dr. Nancy Zahniser in the Department of Pharmacology, University of Colorado School of Medicine. The importance of this ISA to Dr. Zahniser's career and professional growth is that it will continue to (1) minimize her administrative and teaching responsibilities, (2) enhance her focus on drug abuse research, (3) allow her to devote maximal effort toward new research directions initiated during the previous support period and (4) provide increased opportunities to further extend the scope of her research. During the past four years. Dr. Zahniser's lab developed and characterized an in vivo electrochemical method to measure exogenous dopamine (DA) clearance, a measure of DA transporter (DAT) activity, in discrete brain regions of anesthetized rats. Using this approach, they demonstrated that, consistent with behavioral sensitization, the ability of cocaine to inhibit DA clearance is persistently enhanced in nucleus accumbens, but not caudate-putamen, following withdrawal from repeated cocaine administration. Critical to the experiments proposed in the present application, she and her collaborators have recently extended this methodology so that behavior and DAT activity can be measured concurrently in freely-moving rats. In a collaborative project initiated during Dr. Zahniser's sabbatical, human DAT expressed in Xenopus oocytes was shown to be electrogenic, to possess some ion channel-like properties and to be voltage-dependent. Experiments proposed here will explore the latter finding further in the oocyte expression system and in brain slices from rats and D2 DA receptor knockout mice. Specifically, the proposed experiments will test the following hypotheses: (I) that the rate at which exogenous DA is cleared from the extracellular space parallels the rate of endogenous DA clearance, (2) that persistent changes in DAT activity in specific brain regions contribute to the expression of cocaine-induced behavioral sensitization and (3) that D2 DA autoreceptor-induced changes in membrane potential transiently regulate DAT activity. The results of these studies will enhance understanding not only of persistent changes in brain DA systems that contribute to behavioral sensitization but also of mechanisms by which DAT activity is transiently regulated.